Recall the formula for the perimeter (circumference) of a circle.

The circumference $C$ of a circle is given by the formula:

$$C = \pi d$$

where $d$ is the diameter of the circle.

The diameter $d = 21 \text{ cm}$. We will use the approximation $\pi = \frac{22}{7}$.

$$C = \frac{22}{7} \times 21 \text{ cm}$$

$$C = 22 \times \frac{21}{7} \text{ cm}$$ $$C = 22 \times 3 \text{ cm}$$ $$C = 66 \text{ cm}$$

The perimeter of the circle is $\text{66 cm}$.